Jet pumps are employed in many different industrial applications, among them to augment the circulation of cooling water in a boiling water reactor. In such an application, external water pumps are used to develop a high velocity water stream which is delivered by several feed pipes to the several jet pumps located within the reactor shell. The nozzles of each jet pump discharge into the throat of a mixer pipe or the like in which the streams of cooling water are intermixed with the heated water present, driving the mixture out through the bottom of the jet pump diffuser and tail pipe.
It is, on occasion, necessary to stop the water circulation to the jet pump in order to open and repair portions of the external circulation pipeing. In order to prevent reactor water from flowing back through the submerged jet pump nozzles and out of the reactor shell, some arrangement is required for positively closing off the pump nozzles in the interior of the reactor.
The problem described above also exists in early jet pump models, however, such pumps generally have only a single nozzle. Although to close off the sole nozzle of such a jet pump in the interior of a reactor still presents a formidable problem, a number of solutions have been worked out, as shown for example by U.S. Pat. No. 4,043,706. As the jet pump art has developed further, single nozzle jet pumps have been joined by multi-nozzle pumps, for example employing a cluster of five nozzles. In such an arrangement, access to the nozzles is restricted due to the required support structure around the nozzle cluster. This limits the working space available and exacerbates the problem of sealing the nozzles.
Further, in a boiling water nuclear reactor, the introduction of a nozzle closing device, and its subsequent manipulation to bring about the desired sealing of the multiple nozzles, must be carried out from a relatively remote location, typically between about 35 and 65 feet above the nozzles.
Finally, not only is access to the nozzle cluster more restricted in a multi-nozzle jet pump, but each closure device, which usually seals more than one nozzle, has an area and a shape that make it more difficult to maneuver around and through the structure of the jet pump.